1. Field of the Invention
The present invention relates generally to a control system for an automatic power transmission for an automotive vehicle, which has a variable shifting pattern and line pressure. More particularly, the invention relates to an automatic power transmission control for adjusting line pressure depending upon the atmospheric pressure level for providing an enhanced shift feeling irrespective of atmospheric pressure which is variable depending upon altitude, for example.
2. Description of the Background Art
In the modern automotive technologies, automatic power transmissions are electrically or electronically controlled in order to optimize the performance of an internal combustion engine as a prime mover, as well as the vehicle per se. In the advanced technologies, the speed ratio shifting pattern and line pressure are variable depending upon vehicle driving conditions such as engine load, vehicle speed and so forth. In the normal case, the shift patterns and line pressure variation characteristics are determined in terms of a low ground level atmospheric pressure, e.g., lower than 100 m above the sea level. Therefore, optimum speed ratio and driving performance can be obtained at such low ground levels. On the other hand, when a vehicle climbs up to a high ground level area, e.g., 1000 m above the sea level where atmospheric pressure becomes significantly lower than that at the low ground level area, the shifting pattern and line pressure variation characteristics cannot maintain optimum level of shifting and driving performance. That is, at such high altitude area, the shifting patterns and line pressure variation characteristics determined in terms of the low ground level may lead to an increasing of shift shock and lower engine and/or vehicle driving performance. Namely, at high a altitude area, due to the decreasing of induction efficiency of intake air, the engine output torque can be decreased. Holding of the shift pattern and line pressure variation characteristics irrelative to lowering of engine output performance should cause an inappropriate speed ratio selection for degradation of the vehicle driving feeling and, in the worst case, cause an increasing of shift shock.
In order to improve shock problem at a high altitude area, Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-164052 proposes an automatic power transmission control system. In the disclosed system, altitude in traveling is discriminated for selecting one of a plurality of preset shifting patterns. When high altitude is detected, one of the shifting patterns, adapted for high altitude area is selected for performing a shifting up speed ratio at a higher vehicle speed than that in standard or lower ground level shifting pattern. As can be appreciated, the proposed control system may improve drive feeling at high a altitude area. On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-9759 proposes a line pressure control system which lowers line pressure according to a lowering of the atmospheric pressure in order to reduce shift shock at a high altitude area. The proposed system line pressure can be appropriately controlled depending upon altitude for maintaining shift shock at acceptable level.
However, in the former system proposed in the Tokkai Showa 60-164052, though the shifting pattern can be maintained at an optimum level, line pressure is maintained irrespective of the atmospheric pressure which is variable depending upon altitude. Therefore, shift shock can be increased at the higher altitude. On the other hand, the system in the Tokkai Showa 63-9759, though shift shock can be maintained in acceptable level, the shift pattern cannot be appropriate for providing optimum vehicle driving performance.
It may be possible to combine the technologies proposed in the above-mentioned publications to establish a control system which can achieve the appropriate shift pattern selection as well as provide line pressure control depending upon atmospheric pressure. However, if both technologies are combined, systems for performing shift pattern correction and line pressure correction depending upon altitude become too bulky and complicated to make the transmission practically unapplicable for the vehicle. In addition, in the case of a lock-up type automatic power transmission, additional atmospheric pressure dependent correction becomes necessary for controlling the lock-up clutch between engaged and disengaged states. Furthermore, it becomes necessary to control the clutch brake and so forth depending upon engine output torque variation.
Therefore, the automatic power transmission control systems proposed in the prior art are not satisfactory for practical installation on the vehicle.